CHAP, iv.] ELECTRICITY AND MAGNETISM. 249 



definite duration would appear to be drawn out into an 

 elongated streak. Such an elongation was found to be 

 visible when a Leyden jar was discharged through a 

 copper wire half a mile long ; and when the circuit was 

 interrupted at three points, one in the middle and one at 

 each end of this wire, three sparks were obtained, which, 

 viewed in the mirror, showed a lateral displacement, 

 indicating (with the particular rate of rotation employed) 

 that the middle spark took place 115 * 000 of a second 

 later than those at the ends. Wheatstone argued from this 

 a velocity of 288,000 miles per second. But Faraday 

 showed that the apparent rate of propagation of a 

 quantity of electricity must be affected by the capacity 

 of the conductor ; and he even predicted that since a 

 submerged insulated cable acts like a Leyden jar (see 

 Art. 274), and has to be charged before the potential 

 at the distant end can rise, it retards the apparent flow 

 of electricity through it. Professor Fleeming Jenkin 

 says of one of the Atlantic cables, that, after contact 

 with the battery is made at one end, no effect can be 

 detected at the other for two -tenths of a second, and 

 that then the received current gradually increases, until 

 about three seconds afterwards it reaches its maximum, 

 and then dies away. This retardation is proportional 

 to the square of the length of the cable as well as to 

 its capacity and to its resistance ; hence it becomes 

 very serious on long cables, as it reduces the speed 

 of signalling. There is in fact no definite assignable 

 " velocity of electricity." 



A very simple experiment will enable the student to 

 realise the excessively short duration of the spark of a 

 Leyden jar. Let a round disc of cardboard painted 

 with black and white sectors be rotated very rapidly so 

 as to look by ordinary light like a mere gray surface. 

 When this is illuminated by the spark of a Leyden jar it 

 appears to be standing absolutely still, however rapidly 

 it may be turning. A flash of lightning is equally in- 



